Add RHEL.7 guest installation config

1. Add RHEL.7 guest installation config,
include RHEL.7.0.cfg and RHEL.7.ks.

2. Enlarge the installation timeout from 3000 to 4800,
as RHEL.7 graphical installtion and boot needs more packages.

Signed-off-by: Cong Li <coli@redhat.com>
Merge pull request #1789 from CongSmile/RHEL7-guest

Add RHEL.7 guest installation config